The Effect of Reservoir Heterogeneity On Predicted Waterflood Performance In the Dodsland Field

Abstract

Predicted waterflood performance in the Dodsland Viking field has not occurred. Previous study identified that the effects of hydraulic fractures must be taken into account in order to predict performance. The revised predictions are still somewhat optimistic when compared to actual production. The previous work has been extended to account for reservoir layering. Core data has been used to provide a quantitative analysis. Due to extreme permeability contrasts, it is difficult to simulate large ranges in formation permeability as separate layers. Hearn type relative permeability curves were therefore used. With high reservoir heterogeneity, waterflood response may consist only of a levelling of production. This prediction is consistent with actual well performance observed. The methodology used and its limitations are discussed. Additionally, the effects of well spacing and fracture extension are presented. This work has important economic implications for existing and planned waterfloods:Wells which are producing below their economic limit, awaiting predicted waterflood response, can be shut-in. This was used to implement substantial reductions in operating costs.More realistic estimates of waterflood response in highly stratified reservoirs can be made by using core data and Dykstra-Parson ratios prior to implementation. Introduction Outline This paper summarizes a study originally made for an individual unit. Although the results are more broadly applicable, the outline fo(Jows the historical development, under the following headings:Geological DescriptionProduction PerformanceModel ConstructionReservoir ModellingConclusions This work is an extension of work done previously by Carlson and Andrew(1). Although this paper has been written on a standalone basis, further background data may be found in the previous paper. Background The Dodsland field located in south-west Saskatchewan was first discovered in 1953. The location of the field is shown in Figure 1. Light oil (36 ° API) production is obtained from the Cretaceous aged Viking sandstone. Delineation of the field occurred rapidly during the 1950's. Unitization and waterflooding followed in the mid 1960's. A resurgence in development occurred in the la[e 1970's and early (980's due to high oil prices. Development in the mid 1980's was also fuelled by attractive government incentives. Geological Description Lithology The Viking consists of an interbedded shale and sandstone. The sandstone is a poor quality reservoir rock; however, i[s shallow drilling depth and large area have permitted extensive development. The lithology has been described in detail by Tooth et al.(2). The sandstone portions typically consist of sublitharenates which locally grade into siltstones of similar composition. Porosity is intergranular with significant amounts of micro-porosity associated with clay mineral assemblages. Cementing by silica, siderite, pyrite and kaolinite is poor. The more productive areas of the reservoir are comprised of 4.6 to 7.6 m (15 to 25 ft.) thick sandstones with discrete shaly partings. In the less productive areas. The shale partings are dense and the pay consists of thin lenses of sandstone. This layering does not show directly on well logs since it is so fine.

Analytical Model of Waterflood Sweep Efficiency in Vertical Heterogeneous Reservoirs under Constant Pressure

“…The most significant property that affects waterflooding performance is the matrix permeability, and its variation in the vertical direction causes displacing fluid to advance faster in zones of higher permeability and results in earlier breakthrough in such layers. This phenomenon negatively affects volumetric sweep efficiency and leads to low ultimate oil recovery [1][2][3][4][5]. Therefore, it is considerably important to establish a quantitative characterization method for calculating sweep efficiency; by this method, we can determine the criteria of dividing and reorganizing layer series and relieve interlayer interference in stratified waterflooding reservoir development.…”